Electrode structure

Abstract

An electrode structure for use in an electrochemical cell comprises a current collector layer having a current collector surface, an electrode layer having an electrode surface that faces the current

Description

B ACKGROUND Electrode materials can be produced in a free-standing form for subsequent lamination onto a current collector. Poor compatibility between the electrode material and the current collector can lead to poor interfacial stability and the production of contact resistance. Compatibility issues can be alleviated by coating the current collector with a thin layer of a carbon-containing dispersion and drying the thin layer on the current collector, so that the electrically conducting interlayer improves electrical contact between the electrode and the current collector and improving performance of the electrochemical cell that comprises them. The literature describes the use of a dispersion of an organic solvent, binder, and the carbon materials. This can be expensive and may require specialist extraction processes to either capture or recover solvent during the drying stage. Other proposals include the use of a dispersion of water, binder and the carbon materials. The thin layer of carbon-containing dispersion is typically deposited onto the current collector using a slot die system. In the deposition method, the dispersion is forced through a very thin slot, using a pump, to create a coating bead of specific width. When the current collector is contacted with the coating bead, the dispersion is sheared and produces a coating with its thickness influenced by the line speed (the line speed corresponding with the speed at which contact is made between collector and coating bead) and the pump speed, for example. The use of low viscosity dispersions having, for example, single-walled carbon nanotubes as the carbon material, can make it difficult to afford coatings which are very thin with this method. Further, considering that the resulting thin film should have no visible edge defects to maximise performance, such processes limit the loading of material to a minimum of around 0.4 grams per square metre (GSM), or around 1 um thickness. SUMMARY In a first aspect, provided herein is an electrode structure for use in an electrochemical secondary cell. The electrode structure comprises a current collector layer having a current collector surface; an electrode layer having an electrode surface that faces the current collector surface; and an interlayer located on the current collector surface and arranged between the current collector surface and the electrode surface. The interlayer comprises an electrically conductive additive comprising a tubular carbon material as a majority component by weight; and a binder comprising carboxymethyl cellulose. The interlayer has a maximum thickness of 150 nm or less. In some embodiments, the interlayer may have a maximum thickness of 140 nm or less. The present electrode structures have particularly thin interlayers, whose maximum thickness is significantly less than the thicknesses provided by the slot die system. This can lead to a reduced material cost, especially where the carbon material comprises or is singlewalled carbon nanotubes (SWCNTs), because less carbon material is needed to form the thin layer. Nevertheless, the inventors have found that such thin interlayers can show similar stability to thicker layers, so that there is no loss of compatibility e.g. no loss of performance and / or the integrity of the structure is maintained following application of the interlayer. In particular, it has been found that a thin interlayer as described herein can show similar temperature stability to a thicker interlayer, in particular over the range 30-60°C. The present inventors have found that by using an electrically conductive additive which comprises tubular carbon as a majority component, interlayers having improved electrochemical performance may result at reduced thicknesses. In particular, other kinds of carbon material, such as graphite-based allotropes, typically show poorer performance and stability, as well as lower conductivity, which ultimately can hamper power performance, compared to the tubular carbon described herein. In some embodiments, the interlayer may have an average thickness of less than 100 nm. In some embodiments, the interlayer may have a minimum thickness of 15 nm. Such thicknesses can be suitable for alleviating poor compatibility between the electrode material and the current collector, as well as having low levels of visible edge defects. In some embodiments, the interlayer may have a maximum thickness of 90 nm or less, such as 85 nm or less. Such thicknesses can be readily achieved by the methods described herein and are particularly beneficial for the reasons listed above. In some embodiments, the electrically conductive additive may comprise 90 wt% or more of tubular carbon material. In some embodiments, the electrically conductive additive may consist essentially of the tubular carbon material. In such embodiments, the interlayer may not comprise alternative non-tubular electrically conductive additive components. That is, the interlayer may contain substantially no conductive carbonaceous material other than the tubular carbon materials - e.g. the interlayer may not comprise graphite, graphene and / or amorphous carbon (such as carbon black). In some embodiments the interlayer may contain less than 0.5 wt% of conductive carbonaceous materials other than the tubular carbon material, e.g. 0.1 wt% or less, 0.05 wt% or less, or 0.01 wt% or less. In some embodiments, the tubular carbon material may comprise or consist essentially of single-walled carbon nanotubes (SWCNTs) and / or multi-walled carbon nanotubes (MWCNTs). In some embodiments, the tubular carbon material may comprise or consist essentially of single-walled carbon nanotubes (SWCNTs). These kinds of carbon material can show particularly good performance and stability, and high conductivity. In some embodiments, the interlayer may have a uniform thickness. In some such embodiments, the interlayer may have a uniform thickness across its width. The term ‘uniform thickness’ is used herein to describe a low variation in thickness. That is, the interlayer may have a thickness which varies by no more than 40%, such as no more than 35% or no more than 30% across its width. Such interlayers may show no visible edge defects and therefore improved performance. Consistency across the interlayer thickness may also lead to most effective contact between the electrode material and the current collector. In the present invention, the thickness measurements are obtained using a scanning electron microscope (SEM). In some embodiments, the electrode layer may be a polymer, such as a polymer gel, electrode layer. The present electrode structure and methods described elsewhere herein may be particularly effective with free-standing electrodes, which are electrodes formed without the support of a current collector. For example, polymer gel electrodes may have good compatibility with the interlayer described herein. Polymer gel electrodes may be particularly suitable as free-standing electrodes since, if isolated from other components of the electrode structure, they can be of sufficient integrity to be self-supporting and therefore may also be particularly suited for use in the methods discussed elsewhere herein. In some embodiments, the current collector layer may comprise or may be aluminium foil. The interlayer of the present electrode structure may be particularly effective with aluminium foil-based current collectors, especially where the electrode is a cathode. Aluminium foil may also be readily available at a suitably thin thickness for the present applications and methods. In some embodiments, the current collector layer may comprise or may be copper foil. The interlayer of the present electrode structure may be effective with copper foil-based current collectors, especially where the electrode is an anode. Copper foil may also be readily available at a suitably thin thickness of the present applications and methods. In general, the variation in foil thickness along its length (tolerance) of appropriate foils is expected to vary according to the foil thickness. For example, the tolerance is likely to be smaller for thicker foils, and larger for thinner foils. In some embodiments, the tolerance of appropriate foils for use herein may be between around 0.2 and 1 pm, such as between around 0.2 and 0.8 pm, such as between around 0.3 and 0.6 pm, such as around 0.5 pm In some embodiments, the current collector may comprise a second current collector surface and the electrode structure comprises: a second electrode layer having a second electrode surface that faces the second current collector surface; and a second interlayer, the second interlayer being arranged between the second current collector surface and the second electrode surface. Overall, this leads to an electrode structure comprising a current collector layer having a first current collector surface and a second current collector surface, a first electrode layer having a first electrode surface that faces the first current collector surface, and a first interlayer located on the first current collector surface and arranged between the first current collector surface and the first electrode surface, and a second electrode layer having a second electrode surface that faces the second current collector surface, and a second interlayer located on the second current collector surface and arranged between the second current collector surface and the second electrode surface. Such embodiments mean that a single current collector layer can act as a current collector for two electrodes, and thereby maximise the efficiency of the structure when used in an electrochemical cell. At least the first interlayer comprises an electrically conductive additive comprising a tubular carbon material as a majority component by weight and a binder comprising carboxymethylcellulose, and the first interlayer has a maximum thickness of 150 nm or less. In some embodiments, the second interlayer may also comprise an electrically conductive additive comprising a tubular carbon material as a majority component by weight and a binder comprising carboxymethylcellulose, and the second interlayer may have a maximum thickness of 150 nm or less. In some embodiments, the second electrode layer may be the same as the first electrode layer. In some embodiments, the second electrode layer may be different from the first electrode layer. In a second aspect, provided herein is a printing apparatus. The printing apparatus is for producing a layer corresponding to the interlayer described for the first aspect, i.e. a layer that comprises an electrically conductive additive, and a binder, and the layer has a maximum thickness of 150 nm or less. The electrically conductive additive comprises a tubular carbon material as a majority component by weight of the electrically conductive additive. The layer produced by the printing apparatus may be called a primer layer herein. When the primer layer is applied to the current collector, the primer layer and current collector together may be called a primed current collector herein. When an electrode layer is applied to the primed current collector on the available face of the primer layer, the primer layer is termed an interlayer, and may be an interlayer as described for the first aspect. The printing apparatus comprises a first bath, for holding a primer dispersion comprising the electrically conductive additive and the binder, and a first roller set. The first roller set comprises (i) a first engraved roller, for taking up the primer dispersion from the bath onto a surface of the first engraved roller; (ii) a first doctor blade arranged laterally adjacent the first engraved roller, for removal of excess primer dispersion from the surface of the first engraved roller to generate a desired thickness of primer dispersion; and (iii) a first transfer roller arranged above the first engraved roller, for applying the primer dispersion to a current collector surface of a current collector layer arranged on a surface of the first transfer roller. The first engraved roller and the first transfer roller are counter rotatable. The printing apparatus of the second aspect may be particularly suitable for producing a primer layer corresponding to an interlayer as described for the first aspect, and more particularly suitable for producing a primer layer (interlayer) having a maximum thickness of 150 nm which does not show visible edge defects, which is substantially smaller than the thickness of such layers produced by the slot die system. The present apparatus may also be particularly suitable for use with the low viscosity tubular carbon-based primer dispersions needed to produce the interlayer of the electrode structure of the first aspect. In particular, it is found that such apparatus can suitably achieve a coating with less than 0.4 GSM, such as 0.3 GSM or less, such as 0.2 GSM or less. In some embodiments, the first engraved roller may have a surface, which surface contacts the primer dispersion, that comprises or consists of a metal. In some embodiments, the metal may be or may comprise chrome. In some embodiments, the chrome may overlie a different material on which an engraving has been applied. Such materials may be particularly suitable for manufacture of engraved rollers having precise engravings. Such materials may also be particularly suitable for uptake of low viscosity primer dispersions from the bath onto a surface of the engraved roller and without being chemically affected by the primer dispersion. In some embodiments, the first transfer roller may have a surface, which surface contacts the primer dispersion, that comprises or consists of an elastic material. In some embodiments, the elastic material may be or may comprise an elastomer. In some embodiments, the elastic material may be or may comprise rubber. Such elastic, elastomeric or rubber materials may be particularly suitable for transfer of the low viscosity primer dispersions applied to the surface of the engraved roller. The elastic nature of the material may also help apply the necessary pressure to the first engraved roller to transfer the primer dispersion to the first transfer roller without causing defects in either the first engraved roller or the primer dispersion thickness. In some embodiments, the first roller set may comprise a second roller arranged above the first transfer roller to allow a current collector layer to pass between the first transfer roller and the second roller, wherein the second roller has a surface, which surface contacts the primer dispersion, that comprises or consists of a non-elastic material such as a metal. In some embodiments, the metal may be chrome. When the second roller is in contact with the first engraved roller in this way, the second roller may effectively be used as the coating roller. This arrangement may further assist with limiting or preventing visible edge defects with the low viscosity primer dispersions typically used to form the interlayer present in the electrode structure of the first aspect. In some embodiments, such arrangement can reduce the possibility of excess spread of the primer dispersion when the primer dispersion is applied to the current collector. Such spread can occur for example in embodiments in which the first engraved roller applies the primer dispersion directly to the current collector, because the engravings of the first engraved roller carry excess primer dispersion. In typical embodiments in which the transfer rollers do not have such engravings, the presence of such excess primer dispersion is expected to be reduced at least by the presence of the doctor blade, so that the transfer of the primer dispersion to the current collector is not expected to show such excess spread. Accordingly, in some embodiments, the presence of a doctor blade can reduce the spread of solution in a transverse direction. In some embodiments, the presence of a doctor blade can avoid, reduce or minimise the presence of visible edge defects e.g. wavy edges. In some embodiments, the presence of a doctor blade can promote increased edge linearity. In a third aspect, provided herein is a method of producing an electrode structure according to the first aspect. The method comprises applying a primer layer to a current collector surface using a gravure printing process. In some embodiments, the layer of the third aspect corresponds with the interlayer described for the first aspect. In some embodiments, the product of the method is a primed current collector. In some embodiments, the method may include further steps such as applying an electrode layer to form an electrode structure. Once an electrode layer is applied to the primed current collector as discussed elsewhere herein, an electrode structure of the first aspect may be produced. In some embodiments, the gravure printing process is an offset gravure printing process. In some embodiments, the gravure printing process may use the printing apparatus of the second aspect. In some embodiments, the method may comprise applying an electrode layer on the primed current collector to thereby form an electrode structure according to the first aspect. The gravure printing process is a simple process because it can be used to prepare a primer layer having i.a. a maximum thickness of 150 nm, and in the same process the primer layer can be applied to the current collector to form a primed current collector. In a fourth aspect, provided herein is a method of applying a layer to a current collector. The layer of the fourth aspect may be called a primer layer and may correspond with the interlayer described for the first aspect, i.e. comprising an electrically conductive additive comprising a tubular carbon material as a majority component by weight, and a binder, and wherein the primer layer has a maximum thickness of 150 nm or less. The current collector of the fourth aspect corresponds with the current collector described for the first aspect. The method comprises the steps of providing a gravure printing apparatus comprising a first bath and a first roller set, the first roller set comprising a first engraved roller, a first doctor blade arranged laterally adjacent the first engraved roller, and a first transfer roller arranged above the first engraved roller, wherein the first engraved roller and the first transfer roller are counter rotatable; adding a primer dispersion comprising the electrically conductive additive and the binder to the first bath, arranging the first doctor blade at a desired distance from the surface of the first engraved roller, and applying a current collector to a surface of the first transfer roller; taking up a portion of the primer dispersion from the bath onto a surface of the first engraved roller; removing any excess primer dispersion from the surface of the first engraved roller to generate a desired thickness of primer dispersion using the first doctor blade; transferring primer dispersion from the surface of the first engraved roller to a surface of the first transfer roller; and applying the primer dispersion from the first transfer roller to a current collector surface of a current collector layer. In some embodiments, the method uses the printing apparatus of the second aspect. In some embodiments, the method may comprise applying an electrode layer on the primed current collector. In such embodiments, the primer layer is an interlayer, and thereby the method forms an electrode structure, for example an electrode structure according to the first aspect. Accordingly, in some embodiments, provided herein is a method of producing an electrode structure, for example an electrode structure according to the first aspect, comprising a method of applying a layer to a current collector. The layer may be called a primer layer herein. The primer layer may comprise an electrically conductive additive comprising a tubular carbon material as a majority component by weight, and a binder, and have a maximum thickness of 150 nm or less. The method may comprise the steps of providing a gravure printing apparatus comprising a first bath and a first roller set, the first roller set comprising a first engraved roller, a first doctor blade arranged laterally adjacent the first engraved roller, and a first transfer roller arranged above the first engraved roller, wherein the first engraved roller and the first transfer roller are counter rotatable; adding a primer dispersion comprising the electrically conductive additive and the binder to the first bath, arranging the first doctor blade at a desired distance from the surface of the first engraved roller, and applying a current collector to a surface of the first transfer roller; taking up a portion of the primer dispersion from the bath onto a surface of the first engraved roller; removing any excess primer dispersion from the surface of the first engraved roller to a desired thickness using the first doctor blade; transferring primer dispersion from the surface of the first engraved roller to a surface of the first transfer roller; and applying the primer dispersion from the first transfer roller to a current collector surface of a current collector layer. In a fifth aspect, provided herein is an interlayer comprising an electrically conductive additive comprising a tubular carbon material as a majority component by weight, and a binder, and wherein the interlayer has a maximum thickness of 150 nm or less, wherein the interlayer is produced by a method comprising a gravure printing process. In some embodiments, the method may be a method according to the third or fourth aspects. In some embodiments, the interlayer may be part of an electrode structure of the first aspect. In a sixth aspect, provided herein is an electrochemical secondary cell comprising the electrode structure of the first aspect, or an interlayer of the fifth aspect, or a primer layer applied to a current collector by the method of the fourth aspect, or an electrode structure produced according to the method of the third aspect. In some embodiments, the cell may be an alkali metal ion secondary cell, for example a sodium-ion secondary cell or a lithium-ion secondary cell. In some embodiments, the cell may be a lithium-ion secondary cell. In a seventh aspect, provided herein is an electrochemical energy storage device comprising an electrochemical secondary cell according to the sixth aspect. In some embodiments, the electrochemical energy storage device may be a battery. In some embodiments, the electrochemical energy storage device may be a lithium-ion battery. In an eighth aspect, provided herein is a use of the printing apparatus of the second aspect in a method for producing an electrode structure of the first aspect. The method comprises adding a primer dispersion comprising the electrically conductive additive and the binder to the first bath, arranging the first doctor blade at a desired distance from the surface of the first engraved roller; applying a current collector to a surface of the first transfer roller; and rotating the first roller set so that the primer dispersion is taken up on a surface of the first engraved roller, excess primer dispersion is removed from the surface by the first doctor blade, and the primer dispersion is transferred onto a current collector surface by the first transfer roller. Throughout, all of the compositional and method options set out above for the first to eighth aspects apply equally to the other aspects as appropriate. These, and other aspects and embodiments of the invention, are described in further detail herein. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of gravure printing apparatuses. Fig. la shows (i) a kiss-direct arrangement and (ii) a kiss-offset arrangement. Fig. lb shows (i) a direct arrangement and (ii) an offset arrangement. In each case, the leftmost arrangement is the forward arrangement, and the rightmost arrangement is the reverse arrangement. Figs. 2a, 2b, 2c, 2d are photographs of reels of primed current collectors. The primer layer of the reels of Figs. 2a-c have an average GSM across the reel of around 0.3 GSM; the primer layer of the reels of Fig. 2d has an average GSM of 0.17 ±0.075 GSM. Fig. 3 shows surface elemental mapping images from the primed current collector shown in Fig. 2b. Scale bars are 20 pm. From left to right (top) are two SEM images on which mapping was performed; the map for carbon; and the map for oxygen. From left to right (bottom) are the map for sodium; the map for aluminium; and the map for iron. Fig. 4a shows cross-sectional SEM images used to calculate an average thickness of the primed current collector of Figs. 2a-2c. Scale bars = 100 nm. From left to right, arrows indicate: image (i) - 82.35; 75.29 nm; 89.41 nm; 72.94 nm; 70.59 nm; image (ii) - 75.29 nm; 84.71 nm; 70.59 nm; 68.24 nm; 75.29 nm; 105.9 nm; 82.35 nm; 84.71 nm; 89.41 nm; image (iii) - 77.65 nm; 77.65 nm; 82.35 nm; 80.00 nm; 84.71 nm; 77.65 nm; 75.29 nm; 91.76 nm; image (iv) - 80.00 nm; 77.65 nm; 89.41 nm; 72.94 nm; 82.35 nm; 108.2 nm; 72.94 nm; 84.71 nm. Fig. 4b shows energy-dispersive x-ray (EDX) analysis images of the primed current collector of Fig. 4a. (i) is taken at 20000x magnification and shows the distribution of C (light grey) and Al (dark grey); (ii) is a SEM cross-sectional image used to generate (i), (iii) and (iv); (iii) shows the distribution of C (light grey) only; (iv) shows the distribution of Al (light grey) only. Scale bars for each of (ii)-(iv) are 1 pm. Fig. 5a shows the cross-sectional SEM images used to calculate an average thickness of the primed current collector of Fig. 2d. Scale bars = 100 nm. From left to right, arrows indicate: image (i) - 68.24 nm; 58.82 nm; 47.06 nm; 63.53 nm; 61.18 nm; image (ii) - 42.35 nm; 56.47 nm; 42.35 nm; 58.82 nm; 44.71 nm; image (iii) -51.76 nm; 54.12 nm; 63.53 nm; 44.71 nm; 58.82 nm; image (iv) - 58.82 nm; 58.82 nm; 49.41 nm; 44.71 nm; 42.35 nm. Scale bars for each of (ii)-(iv) are 1 gm. Fig. 5b shows EDX analysis images of the primed current collector of Fig. 5a. (i) is taken at 20,000x magnification and shows the distribution of C (light grey) and Al (dark grey); (ii) is a SEM cross-sectional image used to generate (i), (iii) and (iv); (iii) shows the distribution of C (light grey) only; (iv) shows the distribution of Al (light grey) only. Scale bars for each of (ii)-(iv) are 1 gm. Fig. 6a-c show impedance spectra of a laminated symmetric cell carried out at 30°C (Fig. 6a), 45°C (Fig. 6b) and 60°C (Fig. 6c) with the interlayer laminated current collector of Fig. 2d for various times and a gel electrode layer. For each, the left y axis label is -Z” / Qcm2; the x axis label is Z7 Qcnr and the right axis denotes the cycle number and goes to around 37 for the 30°C experiment, to around 15 for the 45°C experiment, and to around 180 for the 60°C experiment. One cycle is around Ih storage. Fig. 7 shows example patterns for the first engraved roller described herein. Fig. 7 i) shows a tri-helical pattern and Fig. 7 ii) shows a hexagonal pattern. DETAILED DESCRIPTION It is to be understood that any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Methods and materials are described herein for use in the present disclosure; other, suitable methods and materials known in the art can also be used. The materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. Methods described herein usually employ ambient temperature of a typical laboratory, which is typically between 20 and 30°C, such as around 25°C, at atmospheric pressure, unless a different condition is defined herein or is more usually employed e.g. for a particular apparatus. Where embodiments discussed herein use the term “comprises” or the like, corresponding embodiments using the term “consists of’ should be considered explicitly disclosed. The present invention generally describes the particularly thin interlayers that exist between a current collector and an electrode, for use in electrochemical secondary cells such as batteries, and their preparation methods. ELECTRODE STRUCTURE The electrode structure of the present invention comprises a current collector layer, an electrode layer, and an interlayer arranged therebetween. In the electrode structure, the function of the current collector layer is to minimise the path length for conduction of electrical current away from the electrode. Accordingly, the current collector layer may be made of any material that is suitable for conducting current. The current collector layer may be any suitable thickness. Suitable current collector layers are known. In some embodiments, the current collector layer may be or may comprise a metal, such as a metal foil. In some embodiments, the current collector layer may be a metal such as a metal foil. The choice of material may be dependent on the electrode (e.g. whether the electrode is an anode or a cathode). In some embodiments, the current collector layer may comprise a single metal (e.g. consist of one metal). In some embodiments, the current collector layer may comprise more than one metal. In some embodiments, the current collector layer may comprise a composite, such as a laminate having a first face comprising a first metal and a second face comprising a second metal, wherein the first and second metals are not the same. In some embodiments, the current collector layer may be a laminate comprising a first face which is a first metal and a second face which is a second metal, wherein the first and second metals are not the same. In some embodiments, the metal may be or may comprise a transition metal or a Group III metal. In some embodiments the metal may be, or may comprise e.g. as part of a laminate, at least one of aluminium, copper, platinum, nickel, molybdenum or tungsten. In some embodiments, the metal may be, or may comprise e.g. as part of a laminate at least one of, aluminium, copper or nickel. In some embodiments, the metal may be, or may comprise e.g. as part of a laminate, at least one of aluminium or copper. In some embodiments, the current collector may be copper. In some particular embodiments, the current collector may be aluminium. In some embodiments, the current collector layer may have a thickness of up to 20 pm, such as up to 18 pm, up to 15 pm, or up to 12 pm. In some embodiments, the current collector layer may have a thickness of at least 1 pm, at least 3 pm, at least 5 pm or at least 7 pm. Any of the foregoing may be combined to form a suitable range, such as between 1 and 20 pm, between 5 and 30 pm, between 3 and 18 pm, between 3 and 12 pm, or between 7 and 12 pm. In some embodiments, the thickness here may refer to the average (mean) thickness. The average (mean) thickness may be measured as described herein for the average (mean) interlayer thickness. The electrode layer functions as an electrode in the electrochemical cell and should contain a material suitable for accepting or producing ions, such as metal ions, for example alkali metal ions such as lithium and / or sodium ions, and particularly lithium ions. The electrode layer may be formed from any suitable material and may be any suitable thickness. Appropriate electrode layers are known. In some embodiments, the electrode layer may be a free-standing electrode layer. A freestanding electrode may be an electrode that has been formed without the support of a current collector layer. A free-standing electrode may be an electrode that, if isolated from other components of the electrode structure, would be of sufficient integrity to be self-supporting. In some embodiments, the electrode layer may be deformable. The deformability may provide for particularly good contact with the interlayer. In some embodiments, the electrode layer may be a solid-state electrode layer. In some embodiments, the electrode layer may be a sintered electrode layer. Sintering may be a convenient method for forming a free-standing electrode. Sintered electrode layers may have a surface roughness suitably accommodated using interlayers as described herein. In some embodiments, the electrode layer may be a cast electrode layer in which the electrode layer is produced by casting the electrode onto the surface of the primed current collector. In some embodiments, the electrode layer may be a dry processed electrode layer, for example a dry processed electrode layer produced without solvent. In some embodiments, the electrode layer may be an extruded electrode layer. In some embodiments, the electrode layer may be a polymer electrode layer. In some embodiments, the polymer electrode layer may be a polymer gel electrode layer. In some embodiments, the polymer electrode layer may be or may comprise the components as set out elsewhere herein. In some embodiments the polymer gel may be compressible, for particularly good contact with the interlayer. In some embodiments, a polymer-gel electrode may be formed from a polymer-electrolyte gel matrix phase, which in turn may be formed from one or more electrolyte components and at least one gelling polymer. Suitable electrolyte components and gelling polymers will be known to a skilled person but may include for example the following. The one or more electrolyte components may include a solvent suitable for use as an electrolyte solvent in a gel electrode, for example an organic solvent. The one or more electrolyte components may include a salt. In some embodiments, the one or more electrolyte components may constitute an electrolyte salt solution or liquid electrolyte. In some embodiments, the one or more electrolyte components may comprise a solvent comprising one or more cyclic or linear carbonate compounds. In some embodiments, the solvent may comprise one or more cyclic carbonate compounds. In some embodiments, the solvent may comprise one or more of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyrolactone. In some embodiments, the solvent may comprise a blend of at least two different compounds, for example at least three or at least four different compounds. In some embodiments the solvent may comprise a blend of at least two different organic carbonate compounds, for example at least three or at least four different organic carbonate compounds. In some embodiments, the electrolyte component!s) may comprise a solvent with low vapor pressure and high flash point to enable safe processing. An example of a solvent fulfilling these criteria is propylene carbonate. Accordingly, the one or more electrolyte components may comprise or consist of propylene carbonate, or a blend of propylene carbonate with one or more of the above listed solvents. In some embodiments, the one or more electrolyte components may comprise an alkali metal salt. The alkali metal of the alkali metal salt may be any suitable alkali metal (Group I of the periodic table). The alkali metal salt may be a lithium, sodium, or potassium salt. The anion of the alkali metal salt may be any suitable anion. Typical anions are known to the skilled person and may be chosen based on the kind of alkali metal. In some embodiments, when the alkali metal is lithium, the anion of the salt may comprise a halogen such as fluorine. Examples include BFT, PFe’, TFST, FST, OTf, DFOB' and TDT. In some embodiments, the one or more electrolyte components may comprise a lithium salt. In some embodiments, the electrolyte may comprise a mixture of at least two different lithium salts. Examples of suitable lithium salts include LiPF(). LiBF4, LiTFSI, LiFSI, LiOTf, LiDFOB and LiTDI. In some embodiments, the salt may be a thermally stable salt. It has been found that LiPFe has relatively low thermal stability relative to other available lithium salts, and accordingly use of LiPFe may be avoided - that is, in some embodiments, the electrolyte component(s) may not include LiPFe. One or more kinds of alkali metal salt may be used in accordance with the present invention. Typically, but not exclusively, when more than one kind of alkali metal salt is used, they may share a common alkali metal. The polymer-electrolyte gel matrix phase may comprise a gel matrix formed by the gelling of one or more gelling polymers when the polymer(s) absorb a liquid electrolyte. The polymer-electrolyte gel matrix phase may therefore comprise a gel comprising the polymer(s) and absorbed liquid electrolyte. The gelling polymer may comprise one or more gelling polymers independently selected from poly(ethyleneglycol dimethacrylate), poly(ethyleneglycol diacrylate), poly(propyleneglycol dimethacrylate), poly(propyleneglycol diacrylate), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), polyurethane (PU), poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(ethylene oxide) (PEO), poly-L-lactic acid (PLA), polystyrene (PS), poly(ethyleneglycol dimethylether), poly(ethyleneglycol diethylether), poly [bi s(m ethoxy ethoxyethoxide)-phosphazene], poly(dimethylsiloxane) (PDMS), polyacene, polydisulfide, polystyrene, polystyrene sulfonate, polypyrrole, polyaniline, polythiophene, polythione, polyvinyl pyridine (PVP), polyvinyl chloride (PVC), polyaniline, poly(3,4-ethylenedioxythiophene) (PEDOT), poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene, poly naphthalene, polyanthracene, polyfuran, polycarbazole, tetrathiafulvalene-substituted polystyrene, ferrocene-substituted polyethylene, carbazolesubstituted polyethylene, polyoxyphenazine, poly(heteroacene), poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide-co-methoxy-polyethyleneglycolacrylate] (Li[PSTFSI-co-MPEGA]), sulfonated poly(phenylene oxide) (PPO), N,N-dimethylacryl amide (DMAAm), lithium 2-acrylamido-2-methyl-l -propane sulfonate (Li AMPS), Poly(lithium 2-Acrylamido-2-Methylpropanesulfonic Acid-Co-Vinyl Triethoxysilane), polyethyleneoxide(PEO) / poly(lithium sorbate), PEO / poly(lithium muconate), PEO / [poly(lithium sorbate)+BF3], PEO copolymer, PEO terpolymer, and NIPPON SHOKUB AI® polymer, or mixtures or co-polymers thereof. In some embodiments, the gelling polymer may comprise one or more gelling polymers independently selected from poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), poly-L-lactic acid (PLA) and polystyrene (PS). In some embodiments, the electrode layer may have a thickness of up to 150 pm, such as up to 145 pm, up to 140 pm, up to 130 pm, up to 120 pm, up to 110 pm or up to 100 pm. In some embodiments, the electrode layer may have a thickness of at least 5 pm, such as at least 10 pm or at least 15 pm. Any of the foregoing may be combined to form a suitable range, such as between 5 to 150 pm, between 5 and 145 pm, between 5 and 140 pm, between 5 and 120 pm, between 5 and 100 pm, between 10 and 150 pm, between 10 and 145 pm, between 10 and 140 pm, between 10 and 130 pm, between 10 and 120 pm, between 10 and 100 pm, between 15 and 150 pm, between 15 and 145 pm, between 15 and 140 pm, between 15 and 130 pm, between 15 and 120 pm, between 15 and 110 pm, or between 15 and 100 pm. In some embodiments, the electrode layer may comprise an electrochemically active material which is a positive active material so that the electrode layer is a cathode layer. In some embodiments, the positive active material may be a lithium transition metal oxide material. In some embodiments, the positive active material may be a lithium transition metal oxide material comprising a mixed metal oxide of lithium and one or more transition metals, optionally further comprising one or more additional non-transition metals. In some embodiments, the positive active material may be a lithium transition metal oxide material comprising lithium and one or more transition metals selected from nickel, cobalt and manganese. In some embodiments, the positive active material may be selected from one or more of lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium nickel cobalt oxide (NCO), aluminium-doped lithium nickel cobalt oxide (NCA), lithium nickel manganese cobalt oxide (NMC), lithium nickel oxide (LNO), lithium nickel manganese oxide (LNMO), lithium iron phosphate (LFP), lithium manganese iron phosphate (LFP) and lithium nickel vanadate (LNV). In some embodiments, the positive active material may be lithium nickel manganese cobalt oxide (NMC), optionally doped with another metal such as aluminium. Such positive active materials may be commercially available or may be manufactured by methods known to the skilled person, for example through the precipitation of mixed metal hydroxide intermediates from a reaction mixture containing different precursor metal salts, followed by calcination to form a mixed metal oxide and optionally lithiation to incorporate lithium into the oxide. The electrochemically active material may be undoped or uncoated, or may contain one or more dopants and / or a coating. For example, the electrochemically active material may be doped with small amounts of one or more metal elements. The electrochemically active material may comprise a carbon coating on the surface of the particles of the material. The electrochemically active material may be a particulate material, i.e. materials made up of a plurality of discrete particles. The particles may comprise primary particles and / or secondary particles formed from the agglomeration of a plurality of primary particles. In some embodiments, the electrochemically active material may make up at least 50 vol% of the electrode layer, based on the total volume of the electrode layer, for example at least 55 vol%, at least 60 vol%, at least 62 vol%, at least 64 vol%, or at least 65 vol%. Suitably, in some embodiments, the electrochemically active material may make up about 64 vol% of the electrode layer. In some embodiments, the electrode layer may contain a conductive additive. In some embodiments, the conductive additive may comprise carbon. In some embodiments, the conductive additive may be present in an amount of from 0.1 vol% to 10 vol% based on the total volume of the electrode layer. For example, the conductive additive may make up from 0.3 vol% to 5 vol%, from 0.4 vol% to 4 vol%, from 0.5 vol% to 3 vol% or from 0.55 vol% to 2.5 vol% of the electrode layer. In some embodiments, the conductive additive may make up 1 vol% or more of the electrode layer, for example from 1 vol% to 3 vol% of the electrode layer. Suitably, in some embodiments, the conductive additive may be present in amounts of about 1.29 vol%, 1.89 vol%, or 2.48 vol% based on the total volume (100 vol%) of the electrode layer. The electrode structure of the present invention may comprise an interlayer located on a surface of the current collector layer and arranged between the current collector surface and a surface of the electrode layer. The interlayer may comprise an electrically conducting material. Accordingly, the interlayer may be electrically conducting, allowing improved electrical contact between the electrode layer and the current collector layer compared to electrode structures having no interlayer. In turn, this can reduce contact resistance and thereby improve cell performance. In some embodiments, an interlayer of the present invention may be porous. In some embodiments, the porosity can provide characteristics imparting particularly good contact between layers. In some embodiments, the interlayer can accommodate any electrode surface roughness and provides an increased contact area, thereby lowering contact resistance and improving cell performance. The interlayer comprises an electrically conductive additive comprising a tubular carbon material as a majority component by weight. In some embodiments, the tubular carbon can contribute to deformability and / or compressibility of the interlayer. The term ‘majority component’ is used herein to define that the component constitutes at least 50 wt% of the total mass of the electrically conductive additive. In some embodiments, the electrically conductive additive may be 60 wt% or more, 70 wt% or more, 80 wt% or more, 90 wt% or more, 95 wt% or more, 99 wt% or more tubular carbon. The component may also constitute at least 50 vol% of the electrically conductive additive, for example 60 vol% or more, 70 vol % or more, 80 vol % or more, 90 vol % or more, 95 vol % or more, 99 vol % or more tubular carbon. The term ‘tubular carbon’ is used herein to define carbonaceous materials having a generally tubular form. Tubular carbon materials may include, but are not limited to, carbon nanotubes (CNTs) - including single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) - as well as carbon fibres such as carbon nanofibers (CNFs) and vapor-grown carbon fibres (VGCFs). In some embodiments, the tubular carbon materials may comprise or may be CNTs. CNTs may have particularly suitable electrical properties. In some embodiments, the tubular carbon materials may comprise or may be SWCNTs. SWCNTs may have particularly suitable electrical properties and thereby may show most benefit in the present invention. In some embodiments, the amount of electrically conductive additive may be balanced between a desirably high amount from the perspective of conductivity, and cost from the perspective of commerciality. Particularly for some of the methods described herein, viscosity may also be a consideration. In some embodiments, the electrically conductive additive may be present in an amount of at least 5 vol% based on the total volume (100 vol%) of the (dried) interlayer, such as at least 10 vol%, at least 15 vol%, or at least 20 vol%. In some embodiments, the electrically conductive additive may be present in an amount of up to 90 vol% based on the total volume (100 vol%) of the interlayer, such as up to 87 vol%, up to 85 vol%, up to 82 vol%, or up to 80 vol%. Any of the foregoing can be combined to form a suitable range, for example the electrically conductive additive can be present in an amount of between 5 and 95 vol% of the (dried) interlayer, such as between 5 and 85 vol%, between 10 and 85 vol%, between 10 and 87 vol%, between 10 and 82 vol%, or between 20 and 80 vol%. In some embodiments, the (dried) interlayer may comprise from 20 wt% to 60 wt% of the electrically conductive additive, based on the total weight (100 wt%) of the interlayer, for example the interlayer may comprise 20 wt% or more, 25 wt% or more, or 30 wt% or more of the electrically conductive additive. The interlayer may comprise 60 wt% or less of the electrically conductive additive, such as 55 wt% or less, or 50 wt% or less. In some embodiments, the (dried) interlayer may comprise from 40 wt% to 80 wt% of the binder, based on the total weight (100 wt%) of the interlayer, for example the interlayer may comprise 40 wt% or more, 45 wt% or more or 50 wt% or more of the binder. The interlayer may comprise 80 wt% or less of the binder, such as 75 wt% or less, or 70 wt% or less. In some embodiments, the interlayer may comprise a greater proportion of electrically conductive additive compared to binder (wt:wt). In some embodiments, the proportion may be between around 20:80 and 45:55, such as between around 20:80 and 40:60, for example around 35:65 (wt:wt). In some embodiments, the proportion may be between around 30:70 and 40:60, for example around 33:67 (wt:wt). The interlayer may comprise a binder that is or comprises carboxymethyl cellulose (CMC). As used herein, CMC refers to an underivatized form of CMC. A CMC derivative may be included in the binder together with CMC. The binder may act to adhere the interlayer to the electrode layer and to the current collector layer, thereby securing the structure. The binder may effectively improve electrical contact and reduce contact resistance between the electrode layer and the current collector layer. CMC is thermoplastic and therefore may be particularly easy to handle and apply to the current collector layer as part of the interlayer. In embodiments where the binder consists of CMC, the above amounts discussed for the binder apply equally to the amounts of CMC in the interlayer. In some embodiments, the interlayer may consist of the binder and the electrically conductive additive. In some embodiments, the interlayer may consist of the binder, which may consist of CMC, and the electrically conductive additive. In such embodiments, the proportion of interlayer which is not the electrically conductive additive may be the CMC, and so the relative proportions of the CMC can be determined from knowledge of the proportion of electrically conductive additive as discussed elsewhere herein. The interlayer may have a maximum thickness of 150 nm or less. In some embodiments, the interlayer may have a maximum thickness of 140 nm or less, such as 130 nm or less, 120 nm or less, 110 nm or less, or 100 nm or less. In some embodiments, the interlayer may have a minimum thickness of 10 nm or more. In some embodiments, the interlayer may have a minimum thickness of 15 nm or more, such as 20 nm or more, 25 nm or more, or 30 nm or more. Any of the foregoing can be combined to form an appropriate range of thickness for the interlayer. Suitable ranges may include, for example, between 10 and 150 nm, between 10 and 140 nm, between 10 and 100 nm, between 15 and 150 nm, between 15 and 140 nm, between 15 and 100 nm, between 20 nm and 140 nm, between 25 nm and 120 nm, between 30 nm and 110 nm, or between 30 and 100 nm. The maximum thickness may be determined using SEM. The maximum thickness as referred to herein means the largest thickness value measured during the measurement of average thickness of the interlayer, as described elsewhere herein. In some embodiments, the interlayer may have an average (mean) thickness of 100 nm or less. In some embodiments, the interlayer may have an average thickness of 95 nm or less, such as 90 nm or less, or 85 nm or less. In some embodiments, the interlayer may have an average thickness of less than 85 nm, such as 80 nm or less, 70 nm or less, or 65 nm or less, such has 55 nm or less. In some embodiments, the interlayer may have an average thickness of at least 10 nm, such as at least 15 nm, at least 20 nm or at least 25 nm. Any of the foregoing can be combined to form an appropriate range of average thickness for the interlayer. Suitable ranges may include, for example, between 10 and 100 nm, between 10 and 95 nm, between 10 and 90 nm, between 15 and 100 nm, between 15 and 95 nm, between 15 and 90 nm, between 20 and 100 nm, between 20 and 90 nm, between 25 and 100 nm, between 25 and 90 nm, or between 25 and 85 nm. In some embodiments, suitable ranges may include, for example, between 10 and 80 nm, between 10 and 70 nm, between 10 and 65 nm, between 15 and 80 nm, between 15 and 70 nm, between 15 and 75 nm, between 20 nm and 80 nm, between 20 and 70 nm, between 20 and 65 nm, between 25 and 80 nm, between 25 and 70 nm, or between 25 and 65 nm. The average thickness may be determined using SEM. The average thickness as referred to herein means the average, i.e. mean, thickness value of at least 3 thickness measurements taken from at least 1 SEM image, such as at least 3 SEM images, such as 4 SEM images, wherein the SEM images are taken from different parts of the interlayer cross-section across its width. In some embodiments, the average thickness may be the average of at least 4 thickness measurements taken from at least 1 SEM image, such as at least 3 SEM images, such as 4 SEM images, wherein the SEM images are taken from different parts of the interlayer cross-section across its width. In some embodiments, the average thickness may be the average of at least 5 thickness measurements taken from at least 1 SEM image, such as at least 3 SEM images, such as 4 SEM images, wherein the SEM images are taken from different parts of the interlayer cross-section across its width. A thickness measurement may be obtained using an SEM image of a cross-section of the interlayer, which may be formed on a current collector surface and / or on an electrode surface, from which a thickness of the interlayer is determined using the cross-sectional image with appropriate software. In some embodiments, the interlayer may have a generally uniform thickness. By ‘uniform thickness’, it is meant that the thickness of the interlayer may vary across its width by no more than 40%, no more than 35% or no more than 33% of the average thickness. For example, an interlayer having an average thickness of 80 nm with a uniform thickness as defined herein may have a thickness of no more than 112 nm and no less than 48 nm across its width, such as a thickness of no more than 108 nm and no less than 52 nm, or no more than 106.4 nm and no less than 53.6 nm. In some embodiments, the uniformity of thickness can be determined by SEM measurements as described herein. Laser profilometry may alternatively be applied. In some embodiments, a surface roughness can be determines using atomic force microscopy (AFM). In embodiments, an interlayer with a uniform thickness may have a thickness that varies across its width by no more than 30%. Such interlayers may have no visible edge defects. By visible edge defects of an interlayer, the present application refers to unevenness of the edge of the interlayer as observed by eye. The edge as referred to herein is an edge bounding the width of the interlayer. In some embodiments, a visible edge defect may have the form of a waviness or curve along the edge of the interlayer. In some embodiments, a visible edge defect may have a different appearance, such as a chip or other non-linearity. In some embodiments, a visible edge defect may be identified by comparing the straightness of the edge of the interlayer with a ruler or similar. In some embodiments, the electrode structure may comprise a current collector layer having a first current collector surface and a second current collector surface, a first electrode layer having a first electrode surface that faces the first current collector surface, and a first interlayer located on the first current collector surface and arranged between the first current collector surface and the first electrode surface, and a second electrode layer having a second electrode surface that faces the second current collector surface, and a second interlayer located on the second current collector surface and arranged between the second current collector surface and the second electrode surface. At least the first interlayer may comprise an electrically conductive additive comprising a tubular carbon material as a majority component by weight and a binder comprising carboxymethylcellulose and may have a maximum thickness of 150 nm or less. In some embodiments, the second interlayer may comprise an electrically conductive additive comprising a tubular carbon material as a majority component by weight and a binder comprising carboxymethylcellulose and has a maximum thickness of 150 nm or less. In some embodiments, the first and / or second interlayers may have some or all of the optional features described elsewhere herein for the interlayer of the electrode structure of the first aspect. In some embodiments, the first and second interlayers may be the same. In some embodiments, the first and second interlayers may have the same composition. In some embodiments, the first and second interlayers may have the same conductive additive. In some embodiments, the first and second interlayers may have the same conductive additive content. In some embodiments, the first and second interlayers may have the same CMC content. In some embodiments, the first and second interlayers may have the same thickness. In some embodiments, the first and second interlayers may be different. In some embodiments, the first and second interlayers may be different e.g. because they have different maximum thicknesses. In some embodiments, the first and second interlayers may be different e.g. because they comprise different amounts of tubular carbon material. Also provided herein is an electrochemical secondary cell comprising the electrode structure as described herein. Also provided herein is an electrochemical energy storage device comprising such electrochemical secondary cell. In some embodiments, the electrochemical energy storage device may be a battery. In some embodiments, the electrochemical energy storage device may be a lithium-ion battery. In some embodiments, the electrochemical secondary cell and / or electrochemical energy storage device may comprise two such electrode structures. For example, each of the anode and cathode may comprise an electrode structure of the present invention and may have an electrolyte arranged between them. In some embodiments, the electrochemical secondary cell and / or electrochemical energy storage device may comprise an electrode structure of the present invention, and a second electrode, or electrode structure, not according to the present invention. METHODS In some embodiments, the electrode structure of the present invention may be obtained or may be obtainable by a method comprising a gravure printing process. In some embodiments, the method may comprise applying a layer, which may correspond with the interlayer described for the first aspect, to a current collector surface described for the first aspect using the gravure printing process. Accordingly, in some embodiments, a layer corresponding with an interlayer as described herein may be obtained or may be obtainable by a method comprising a gravure printing process. Such layers (not having the electrode layer formed thereon) may be called primer layers, and, when applied to a current collector, the current collector with primer layer may be called a primed current collector. In some embodiments, the process may be a kiss-direct process. In some embodiments, the process may be a kiss-offset process. In some embodiments, the process may be a direct process. In some embodiments, the process may be an offset process. The present invention also provides apparatuses suitable for producing a primer layer, which corresponds with an interlayer as described herein, and / or suitable for use in the methods described herein. Suitable apparatuses that may optionally be used in the present methods are shown in Figs, la (i) and (ii) and lb (i) and (ii), used in the following explanation but without limitation. The printing apparatus of the present invention comprises a first bath and a first roller set. The first roller set comprises a first engraved roller 3, a first doctor blade 5 arranged laterally adjacent the first engraved roller. For preferred methods, the first roller set of the present apparatus also comprises a first transfer roller 11 arranged above the first engraved roller. The first engraved roller 3 and the first transfer roller 11 are counter rotatable. In the present processes, a primer dispersion 7 is provided and applied to the bath 1. The size, shape and material of the bath 1 are not particularly limited, as long as the bath 1 is suitable for holding the primer dispersion 7 and for the uptake of the primer dispersion 7 by the engraved roller 3 as discussed further below. In some embodiments, the primer dispersion 7 may contain the components necessary for producing a primer layer corresponding to an interlayer as described herein for the first aspect. For example, the primer dispersion 7 may comprise at least an electrically conductive additive and a binder. In some embodiments, the primer dispersion 7 may be provided as an aqueous solution or mixture. Appropriate amounts of these in the primer dispersion 7 are based on the final quantities required for the interlayer. In some embodiments, the primer layer may be dried on the current collector in-line. In some embodiments, drying may be carried out by heating at a desired temperature for a desired period of time. In some embodiments, heating may be carried out at up to 95°C, such as up to 90°C. In some embodiments, heating may be carried out at at least 70°C, such as at least 75°C. Suitable heating temperatures may be around 85°C. In some embodiments, heating may be carried out at a single temperature, or may be stepped. In some embodiments, drying may be carried out in air. In some embodiments, drying may be carried out in air having any suitable velocity, such as at least 2 m / s, at least 3 m / s, at least 4 m / s, at least 5 m / s or at least 6 m / s. In some embodiments, drying may be carried out in air having a velocity of up to 15 m / s, such as up to 14 m / s, up to 13 m / s or up to 12 m / s. In some embodiments, the velocity of the air may be between 2 and 15 m / s, between 3 and 13 m / s or between 5 and 12 m / s. The period of heating may be sufficient to dry the primer layer to form the (dried) primed current collector and may typically be influenced by the heating temperature and / or by the velocity of air. In some embodiments, the (dried) primed current collector may be wound as a reel. In some embodiments, a primer layer may be applied to both sides of the current collector. In some embodiments, the primer layers may be formed from the same primer dispersion. In some embodiments, the primer layers may be formed from different primer dispersions. In these embodiments, the options described herein can apply to one or both of the primer layers. In some embodiments, the electrode layer may be applied directly to the primed current collector. In some embodiments, such as those in which the electrode layer is a gel electrode, the gel electrode may be formed directly onto the primed current collector. In some embodiments, the primer dispersion may comprise or may consist of the electrically conductive additive, the binder, and a dispersant. The dispersant may typically be a liquid. In some embodiments, the dispersant may consist of a single liquid. In some embodiments, the dispersant may comprise more than one liquid, so that it is a mixture of dispersants. In some embodiments, the dispersant may comprise water, so that the primer dispersion is an aqueous dispersion. In some embodiments, the dispersant may comprise water as a majority dispersant. In some embodiments, the dispersant such as the dispersant mixture may comprise an alcohol, such as a short-chain alcohol, such as methanol, ethanol, propanol, or butanol. In some embodiments, the dispersant may consist of water. In some embodiments, the primer dispersion may comprise at least 0.1 wt% electrically conductive additive based on total weight (100 wt%) of the primer dispersion, such as at least 0.2 wt%, or at least 0.3 wt%. In some embodiments, the primer dispersion may comprise up to 5 wt% electrically conductive additive based on total weight (100 wt%) of the primer dispersion, such as up to 4 wt%, up to 3 wt% or up to 2 wt% such as around 1 wt%. Any of the foregoing may be combined to form a suitable range, such as between 0.1 and 5 wt%, between 0.1 and 4 wt%, between 0.1 and 3 wt%, between 0.1 and 4 wt%, between 0.1 and 5 wt%, between 0.2 and 4 wt%, or between 0.3 and 2 wt%. In some embodiments, the primer dispersion may comprise at least 0.1 wt% binder based on total weight (100 wt%) of the primer dispersion, such as at least 0.2 wt%, at least 0.3 wt%, at least 0.4 wt% or at least 0.5 wt%. In some embodiments, the primer dispersion may comprise up to 5 wt% binder based on total weight (100 wt%) of the primer dispersion, such as up to 4 wt%, up to 3 wt% or up to 2 wt% such as around 1 wt%. Any of the foregoing may be combined to form a suitable range, such as between 0.1 and 5 wt%, between 0.1 and 4 wt%, between 0.1 and 3 wt%, between 0.1 and 4 wt%, between 0.1 and 5 wt%, between 0.2 and 4 wt%, between 0.3 and 2 wt%, between 0.4 and 5 wt%, or between 0.5 and 2 wt%. In some embodiments, the primer dispersion may comprise at least 90 wt% solvent based on total weight (100 wt%) of the primer dispersion, such as at least 92 wt%, at least 94 wt%, at least 96 wt%, or at least 98 wt%. In some embodiments, the primer dispersion may comprise up to 99.8 wt% solvent based on total weight (100 wt%) of the primer dispersion, such as up to 99.6 wt%, up to 99.4 wt%, up to 99.2 wt% such as around 99 wt%. Any of the foregoing may be combined to form a suitable range, such as between 90 and 99.8 wt%, between 90 and 99.4 wt%, between 92 and 99.6 wt%, between 94 and 99.4 wt%, or between 98 and 99.8 wt%. In some embodiments, the primer dispersions used herein may have a viscosity of up to 250,000 cPs at a shear rate of 1 s’1 measured at 25°C, such as up to 200,000 cPs, up to 200,000 cPs, up to 100,000 cPs or up to 75,000 cPs. In some embodiments, the primer dispersions used herein may have a viscosity of at least 1,000 cPs at a shear rate of Is'1 measured at 25°C, such as at least 1,500 cPs, at least 2,500 cPs, at least 3,500 cPs, or at least 4,500 cPs. Any of the foregoing can be combined to make a suitable range, such as between 1,000 and 250,000 cPs, between 1,500 and 200,000 cPs, between 2,500 and 75,000 cPs, between 3,500 and 100,000 cPs, or between 4,500 and 75,000 cPs at a shear rate of Is’1 measured at 25°C. In some embodiments, the primer dispersions used herein may have a viscosity of up to 15,000 cPs at a shear rate of 10s'1 measured at 25°C, such as up to 10,000 cPs, up to 9,000 cPs, up to 8,000 cPs or up to 7,000 cPs. In some embodiments, the primer dispersions used herein may have a viscosity of at least 100 cPs at a shear rate of 10s’1 measured at 25°C, such as at least 200 cPs, at least 300 cPs, at least 400 cPs, or at least 500 cPs. Any of the foregoing can be combined to make a suitable range, such as between 100 and 15,000 cPs, between 200 and 10,000 cPs, between 300 and 8,000 cPs, between 400 and 10,000 cPs, or between 500 and 7,000 cPs at a shear rate of 10s'1 measured at 25°C. In some embodiments, the primer dispersions used herein may have a viscosity of up to 6,500 cPs at a shear rate of 15s’1 measured at 25°C, such as up to 6,000 cPs, up to 5,500 cPs, or up to 5,000 cPs. In some embodiments, the primer dispersions used herein may have a viscosity of at least 50 cPs at a shear rate of 15s'1 measured at 25°C, such as at least 100 cPs, at least 150 cPs, at least 200 cPs, or at least 250 cPs. Any of the foregoing can be combined to make a suitable range, such as between 50 and 6,500 cPs, between 150 and 5,500 cPs, between 100 and 5,000 cPs, between 250 and 6,500 cPs, or between 250 and 5,000 cPs at a shear rate of 15s'1 measured at 25°C. In some embodiments, the primer dispersions used herein may have a viscosity of up to 2,000 cPs at a shear rate of 100s’1 measured at 25°C, such as up to 1,750 cPs, up to 1,500 cPs, or up to 1,000 cPs. In some embodiments, the primer dispersions used herein may have a viscosity of at least 10 cPs at a shear rate of 100s'1 measured at 25°C, such as at least 20 cPs, at least 30 cPs, at least 40 cPs, or at least 50 cPs. Any of the foregoing can be combined to make a suitable range, such as between 10 and 2,000 cPs, between 20 and 1,000 cPs, between 30 and 1,500 cPs, between 40 and 1,750 cPs, or between 50 and 1,000 cPs at a shear rate of 100s’1 measured at 25°C. Viscosity measurements may be made on any suitable device, such as a rheometer. An exemplary rheometer is HR30 Hybrid Rheometer from TA(RTM) Instruments. In general, the rollers of the apparatus may be of any suitable size, and the mechanism of rotation may be any that is appropriate. The rollers may be generally cylindrical. The engraved roller 3 has an engraved surface i.e. it is not smooth. An engraved surface may enhance the ability of the roller 3 to take up primer dispersion 7 from the bath 1. The pattern of engraving of the engraved roller 3 is not particularly limited. The pattern may be applied to the engraved roller 3 by any suitable process. The method of application may be determined by the desired geometry. For example, the pattern may be produced by an engraving process. In some embodiments, the pattern may be produced by a laser engraving process. In some embodiments, a pattern including perpendicular cell walls, for example, may be produced most effectively or easily by laser engraving. In some embodiments, the pattern may be produced by an etching process. In some embodiments, the pattern may be produced by a mechanical engraving process. In some embodiments, a pattern including angled or trapezoidal cell shapes may be produced most effectively or easily by a mechanical engraving or etching process. The transfer roller 11 does not necessarily have such engraving or pattern, and in some embodiments may not have such engraving or pattern, and in some embodiments the surface may be approximately smooth. In some embodiments, the transfer roller 11 may have a step or lip extending concentrically around the edges of the roller 11. Such step or lip on the first transfer roller 11 may in some embodiments prevent, minimise or reduce excess spread of primer dispersion. In some embodiments, a smooth surface of the first transfer roller 3 may contribute to uniformity, and the step or lip may prevent, minimise or reduce the primer dispersion 7 from spreading beyond a desired width, and / or any excess fluid that reaches the step or lip can spill away to a lower position, where it does not come into contact with the current collector 9 thus maintaining the desired primer dispersion width. An apparatus suitable for use in a kiss-direct process is shown in Fig. la (i). The current collector 9 is applied to a surface of an engraved roller 3 without pressure from another roller. The engraved roller 3 is positioned so that, when the roller rotates, it uptakes primer dispersion 7 from a bath 1 and transfers the primer dispersion 7 directly to the surface of the current collector 9. A doctor blade 5 is arranged adjacent the roller 3 at a position that allows the production of a primer layer having a desired thickness by removing any excess primer dispersion 7 from the surface of the roller 3. In particular, it can be seen that, in some embodiments, the engraved roller 3 takes up an amount of primer dispersion 7 that is thicker than desired, i.e. which will form a primer layer that is thicker than wanted. A doctor blade 5 may be arranged to reduce the amount (thickness) of primer dispersion 7 to generate a thinner primer layer if desired. In this way, the doctor blade 5 controls the thickness of the primer dispersion 7 that is transferred to the surface of the current collector 9. An apparatus suitable for use in a kiss-offset process is shown in Fig. la (ii). A current collector 9 is applied to the surface of the transfer roller 11 which is positioned above the engraved roller 3. An engraved roller 3 is positioned so as to uptake primer dispersion 7 from a bath 1 as in the kiss-direct process. However, instead of transferring the primer dispersion 7 to the surface of a current collector 9, the engraved roller 3 instead applies it to the surface of a transfer roller 11. The transfer roller 11 in turn applies the primer dispersion 7 to the surface of a current collector 9. This creates a primed current collector. As shown by the arrows in Fig. la (ii), the engraved roller 3 rotates in a direction opposite to that of the transfer roller 11. An apparatus suitable for use in a direct process is shown in Fig. lb (i). This is similar to the kiss-direct apparatus, except that a second roller 13 is arranged above an engraved roller 3 and a current collector 9 is fed between an engraved roller 3 and the second roller 13. In some embodiments, the current collector 9 contacts the surfaces of both the engraved roller 3 and the second roller 13. In this way, the second roller 13 can be used to apply pressure to the current collector 9 and press it against the surface of the engraved roller 3, thereby allowing the current collector 9 to more effectively take up the primer dispersion 7. The arrangement of the engraved roller 3, bath 1, and doctor blade 5 are as for the kiss-direct process. An apparatus suitable for use in an offset process is shown in Fig. lb (ii). This is similar to the kiss-offset apparatus, except that a second roller 13 is arranged above a transfer roller 11 and a current collector 9 is fed between a transfer roller 11 and the second roller 13. In some embodiments, the current collector 9 contacts the surfaces of both the transfer roller 11 and the second roller 13. In this way, the second roller 13 can be used to apply pressure to the current collector 9 and press it against the surface of the transfer roller 11, thereby allowing the current collector 9 to more effectively take up the primer dispersion 7. The arrangement of the engraved roller 3, transfer roller 11, bath 1, and doctor blade 5 are as for the kiss-offset process. As noted elsewhere herein, such apparatus in some embodiments may be particularly suitable for producing layers having particularly straight edges, as well as giving good control over doctor blade pressure thereby achieving particularly thin primer layer (and therefore interlayer) thicknesses. In typical embodiments, the doctor blade 5 may contact the roller 3 and a sufficient, minimum force may be applied to ensure even pressure across the width so that the pattern is filled with the primer dispersion 7. If required, the doctor blade 5 can be moved closer to, or further away from, the roller 3 to achieve a desired thickness of primer dispersion 3 on the current collector 9. In some embodiments, the positioning of the doctor blade 5 may be a manual process. In some embodiments, the positioning of the doctor blade 5 can be automated e.g. using a pneumatic pressure system. In some embodiments of any of the above, the process may be a forward process. In such embodiments, the roller 3,11 that applies the primer dispersion 7 to the current collector 9 surface may rotate in the same direction of travel of the current collector 9 as shown by the arrows of Figs, la and b. In some embodiments of any of the above, the process may be a reverse process. In such embodiments, the roller 3,11 that applies the primer dispersion 7 to the current collector 9 surface may rotate in the opposite direction as the direction of travel of the current collector 9 as shown by the arrows of Figs, la and b. In some embodiments, a reverse process can provide greater uniformity of coating, and / or greater evenness of distribution of primer dispersion 7 as applied to the current collector 9, compared to a forward process. In some embodiments of any of the above, the first engraved roller 3 comprises a metal at least at a surface thereof. In some embodiments, the surface metal may be or comprise one particularly suitable for having a pattern applied e.g. by an engraving process. In some embodiments, the surface metal may be chemically unreactive, or has low reactivity e.g. is resistant to corrosion. This may be particularly suitable for avoiding possible reactions with the primer dispersion 7. In some embodiments, the surface may comprise chrome. In some embodiments, the first engraved roller 3 may comprise engraved copper covered with chrome. In some embodiments, the first engraved roller 3 may have a body made of a material which is different to the material of the surface. In some embodiments, the body may be stainless steel. In some embodiments, the first engraved roller 3 may comprise stainless steel coated with engraved copper covered with chrome. The first engraved roller may be engraved with any suitable pattern. In some embodiments, the pattern may be a regular repeating pattern. In some embodiments, the pattern may be irregular or random. The shape of the patterning is not particularly limited. In some embodiments, the pattern may comprise or consist of a tri-helical cell pattern, such as is shown in Fig. 7 i). In some embodiments, the pattern may comprise or consist of a hexagonal pattern, such as is shown in Fig. 7 ii). In some embodiments, the engraved roller 3 may have a pattern applied to the circumferential surface and the volume of the pattern across the circumferential surface (cell volume) can be any suitable volume. For example, the cell volume may be at least 20, at least 30, at least 40 or at least 45 cc / m2. In some embodiments, the cell volume may be up to 80, up to 70, or up to 60 cc / m2. Any of these end-points may be combined to form a suitable range, for example between 20 and 80 cc / m2, between 30 and 60 cc / m2, between 40 and 70 cc / m2 or between 45 and 60 cc / m2. In some embodiments of the above, the first transfer roller 11 may be or may comprise an elastic material at least at a surface thereof. The elastic material may be or may comprise an elastomer, such as rubber. Elastic materials may be particularly useful for the transfer roller 11 because they can allow additional pressure to be applied to enhance uptake of the primer dispersion 7 and to enhance subsequent application to the current collector 9. An elastic material such as rubber may have wetting properties and / or an appropriate coefficient of friction and / or a sufficiently smooth and even surface, any or all of which may allow for tailoring to or particular suitability for use with the particular primer dispersion 7 being used, to assist in a consistent transfer and uniform application. Additionally or alternatively, an elastic material such as rubber may be relatively low cost and / or may be sufficiently durable. In some embodiments, the rubber may be or may comprise an ethylene propylene diene. In some embodiments, the elastic material may be or may comprise a material other than rubber, such as silicone, polyurethane or another polymer. In some embodiments, the elastic material may be or may comprise a material harder than ethylene propylene diene, such as polyurethane, for ease of surface preparation and uniformity of coating. In some embodiments, the elastic material may be present on the surface of the first transfer roller 11. In some embodiments, a main body of the first transfer roller may be a material other than an elastic material, such as a metal. An exemplary metal may be stainless steel. Accordingly, in some embodiments, the elastic material may be present on the surface of the first transfer roller 11 and a main body of the first transfer roller may be a metal such as stainless steel. In some embodiments, the first transfer roller 11 may comprise a step or lip at a circumferential edge thereof. In some such embodiments, the step or lip can reduce, minimise or prevent formation of visible edge defects and in particular wavy lines. In some embodiments, a step or lip may be present at each circumferential edge of the first transfer roller 11. In some embodiments of the above, the second roller 13 may be or may comprise a nonelastic material at least at a surface thereof. In some embodiments, the second roller 13 may be or may comprise a non-elastic material throughout e.g. including the surface and a main body thereof. In some embodiments, the non-elastic material may be different at a surface thereof compared to the main body thereof. The non-elastic material may be a metal. In some embodiments, the metal may be chrome. In some embodiments, the chrome may be polished chrome. In some embodiments, the chrome or polished chrome may have improved durability compared to some other metals. In some embodiments, the chrome or polished chrome may be present on the surface of the second roller 13. In some such embodiments, the main body of the second roller 13 may be a material other than chrome or polished chrome, for example a different metal which may be less expensive. In some embodiments, such different metal may be stainless steel. In some embodiments, the second roller 13 may act to support the current collector 9 during application of the primer dispersion 7. Accordingly, in typical embodiments, the second roller 13 may contact the current collector 9. In some embodiments, the second roller 13 may not contact the first transfer roller 11. In some embodiments, the second roller 13 may contact the first transfer roller 11. In some such embodiments, the second roller 13 may be a movable roller and the first transfer roller 11 may be a stationary roller, so that the second roller 13 may be capable of applying pressure to the current collector 9 thereby helping to maintain tension in the current collector 9. Accordingly, the gravure printing process of the present invention may comprise a step of providing a gravure printing apparatus comprising providing a first bath 1 and a first roller set, the first roller set comprising a first engraved roller 3, a first doctor blade 5 arranged laterally adjacent the first engraved roller 3, and a first transfer roller 11 arranged above the first engraved roller 3, wherein the first engraved roller 3 and the first transfer roller 11 are counter rotatable. In some embodiments, the amount of primer dispersion 7 carried by the first transfer roller 11 can be adjusted e.g. by changing the distance of or pressure applied by the doctor blade 5, and / or by adjusting the position of the first engraved roller 3 and / or of the first transfer roller 11 with respect to each other and / or with respect to the bath 1. Such adjustments may affect the amount of primer dispersion 7 applied to the current collector 9 and therefore the thickness of the layer. In some embodiments, a rotation speed of the first engraved roller 3 and / or the first transfer roller 11 and / or the second transfer roller 13 can be adjusted. Such adjustments may affect the amount of primer dispersion 7 applied to the current collector 9 and therefore the thickness of the layer. In some embodiments, a gap between the first transfer roller 11 and the second transfer roller 13 may be controlled by placing blocks between roller mounts. Such gap may be controlled so as to provide as close to 100% of transfer of primer dispersion 7 to current collector 9 as possible, while minimising, reducing or eliminating visible edge defects as discussed elsewhere herein. In some embodiments, the gap may be altered during production, and in some embodiments, the gap may be altered between reels. In some embodiments, the cell volume and rotation speed of the first engraved roller 3 may be calibrated to deliver a particular and consistent amount of primer dispersion 7 applied to the current collector 9. In some such embodiments, the amount of primer dispersion 7 applied to the current collector may not be affected by the line speed. In some embodiments, increasing the rotation speed of the first engraved roller 3 can increase the amount of primer dispersion 7 transferred to the first transfer roller 11. Accordingly, in some embodiments, the rotation speed of the first engraved roller 3 can be adjusted during production to accommodate e.g. small changes in temperature. In some embodiments, the tension of the current collector 9 may be at least 1 N, such as at least 2 N, at least 5 N or at least ION. In some embodiments, the tension of the current collector 9 may be up to 60 N, such as up to 55 N, or up to 50 N. Any of the foregoing may be combined to form a suitable range, so that the tension of the current collector in the method may be between 1-60 N, such as between 5-55 N, or between 10-50 N. In some embodiments, the cell volume of the first engraved roller 3 may be at least 20 cc / m2, such as at least 25 cc / m2, at least 30 cc / m2, or at least 40 cc / m2. In some embodiments, the cell volume of the first engraved roller 3 may be up to 80 cc / m2, such as up to 75 cc / m2, up to 70 cc / m2, or up to 60 cc / m2. Any of the foregoing may be combined to form a suitable range, such as between 20-80 cc / m2, between 30-70cc / m2, or between 40-60 cc / m2. In some embodiments, the rotation speed of the second transfer roller 13 may be at least 0.5 m / min, at least 0.6 m / min or at least 0.7 m / min. In some embodiments, the rotation speed of the second transfer roller 13 may be up to 10 m / min, such as up to 8 m / min, or up to 6 m / min. Any of the foregoing can be combined to form a suitable range, such as between 0.5-10 m / min, between 0.5-8 m / min, or between 0.7-6 m / min. In some embodiments, the rotation speed of the first engraved roller 3 and / or the first transfer roller 11 can be up to +1- 50% of the rotation speed of the second transfer roller 13. Accordingly, in some embodiments, the rotation speed of the first engraved roller 3 and / or the first transfer roller 11 may be at least 0.25 m / min, such as at least 0.35 m / min, at least 0.5 m / min, at least 0.7 m / min, at least 0.75 m / min or at least 1.05 m / min. In some embodiments, the rotation speed of the first engraved roller 3 and / or the first transfer roller 11 may be up to 15 m / min, such as up to 12 m / min, up to 10 m / min, up to 8 m / min, up to 5 m / min, up to 6 m / min, up to 4 m / min, up to 3 m / min or up to 0.9 m / min. Any of the foregoing may be combined to form a suitable range, such as between 0.25-15 m / min, between 0.5-10 m / min, between 0.75-5 m / min, between 0.25-12 m / min, between 0.5-8 m / min, between 0.75-4 m / min, between 0.35-0.9 m / min, between 0.7-6 m / min, or between 1.05-3 m / min. In some embodiments, a gap between the first engraved roller 3 and the first transfer roller 11 may be set so as to allow a consistent pressure across the width of the rollers 3,11. In some embodiments, the rollers 3,11,13 may be fixed in position using bolts. In some embodiments, a gap between the first transfer roller 11 and the second transfer roller 13 may be at least 0.01 mm, at least 0.03 mm or at least 0.05 mm. In some embodiments, a gap between the first transfer roller 11 and the second transfer roller 13 may be up to 0.5 mm, such as up to 0.3 mm or up to 0.1 mm. Any of the foregoing can be combined to form a suitable range, such as between 0.01-0.5 mm, between 0.03-0.3mm, or between 0.05-0.1 mm. The method may further comprise adding the primer dispersion 7 comprising the electrically conductive additive and the binder to the first bath 1, arranging the first doctor blade 5 at a desired distance from the surface of the first engraved roller 3, and applying the current collector 9 to the surface of the first transfer roller 11. The order of these is not particularly limited. The method may further include taking up a portion of the primer dispersion 7 from the bath 1 onto the surface of the first engraved roller 3. In some embodiments, the primer dispersion 7 portion may be taken up from the bath 1 by rotating the engraved roller 3 and permitting a part of the surface of the engraved roller 3 to pass through the primer dispersion 7 held in the bath 1. In some embodiments, the portion of primer dispersion 7 may remain in contact with the surface of the engraved roller 3 as it rotates through and out of the bath 1. The method may further include removing any excess primer dispersion 7 from the surface of the first engraved roller 3 to a desired thickness using the first doctor blade 5. That is, the doctor blade 5 may remove excess primer dispersion 7 from the surface of the first engraved roller 3 to a desired thickness for transfer to the current collector 9 as the first engraved roller 3 rotates. The method may further include transferring primer dispersion 7 from the surface of the first engraved roller 3 to a surface of the first transfer roller 11. This transfer occurs after the doctor blade 5 has removed the excess primer dispersion 7. In some embodiments, the first transfer roller 11 may be in contact with the first engraved roller 3. This may facilitate the transfer of the primer dispersion 7 from the engraved roller 3 to the transfer roller 11. Typically, the transfer occurs by counter-rotating the first engraved roller 3 and the first transfer roller 11. The method may further include applying the primer dispersion 7 from the first transfer roller to a current collector surface of a current collector layer 9. In some embodiments, the current collector 9 may be provided so that it is in contact with the surface of the first transfer roller 11. In some embodiments, a second roller 13 is arranged above the first transfer roller 11 and the current collector 9 may be fed through between the first transfer roller 11 and the second roller 13. The direction in which the current collector 9 is fed is not particularly limited and may be in the same direction as the direction of rotation of the first transfer roller 11 or in the opposite direction to the direction of rotation of the first transfer roller 11. In some embodiments, the current collector 9 may contact the surface of both the first transfer roller 11 and the second roller 13. In some embodiments, the method may comprise adjusting the distance of the doctor blade 5 from the surface of the first engraved roller 3. This adjustment is for optimising the thickness of the primer layer formed on the current collector 9. In some embodiments, the method may include a step of preparing the current collector 9 for use in the printing method. Preparing the current collector 9 may comprise removing oil from the current collector surface and / or improving surface wettability of the current collector compared to before the preparing step. In some embodiments, removing oil from the current collector surface may comprise a pre-treatment such as oxygen plasma treatment followed by corona treatment. In some embodiments, oxygen plasma treatment may be carried out under vacuum. In some embodiments, the results of such preparation can be assessed using a Dyne test as detailed elsewhere herein. In some embodiments, the method may comprise one or more, such as all, of the steps of: Setting up equipment Aligning rollers 3,11,13 Assessing current collector 9 tension (e.g. to check that the current collector does not wrinkle) - Preparing the current collector - Coating the current collector with primer dispersion - Drying primer dispersion - Winding the primed current collector into a reel. In some embodiments, the drying can occur at a temperature of up to 95°C, such as up to 93°C or up to 90°C. In some embodiments, the method may be particularly suitable for producing primer layers (and therefore interlayers) having particularly linear (straight) edges, as well as giving good control over doctor blade pressure thereby achieving a particularly thin interlayer thickness in a final electrode structure. In some embodiments, any of the methods described herein may comprise a step of applying an electrode layer on the primer layer. In some embodiments, such method may therefore be a method of forming an electrode structure as described herein. In some embodiments, the electrode layer may be formed on the primer layer after the primer layer is formed on the current collector i.e. the electrode layer is formed on the primer layer of a primed current collector. The inventors have found that in some embodiments the offset process can produce a layer having edges with high linearity as well as giving good control over doctor blade pressure thereby achieving particularly thin layer thicknesses. In some embodiments, straighter (more linear) layer edges can be achieved using the offset process, compared to other processes such as the direct process. Linearity can be assessed e.g. by the methods described herein such as use of a ruler. Without wishing to be bound by theory, it is believed that in the kiss-direct process (Fig. la (i)), excess primer dispersion can become located in engravings of the engraved roller, forming excess primer dispersion. In embodiments where insufficient doctor blade pressure can be applied, and / or if the surface tension and volume of the primer dispersion are not sufficiently aligned with rotation speed, such excess primer dispersion may become applied to the current collector in an area beyond an intended application area. Accordingly, while this process can achieve high uniformity of surface and thickness over the bulk of the primer layer, in some embodiments visible edge defects (wavy edges) may arise due to spreading of the excess solution in a transverse direction. In contrast, in an offset procedure, the primer dispersion is applied to the current collector through a roller which does not have such engravings. Thus, the excess primer dispersion is not typically applied to the current collector and thereby reducing the propensity for visible edge defects. In some embodiments, occurrence of wavy edges can also be reduced / minimised / avoided by: - Providing a precise and stable side-to-side positioning of the rollers 3,11,13; and / or - Monitoring alignment of the current collector 9, to avoid drifting and therefore edge uniformity caused by e.g. variations in thickness of the current collector 9; and / or - Preventing accumulation or build-up of primer dispersion 7 on the surfaces of the rollers 3,11,13. Also provided herein is an electrochemical secondary cell comprising the interlayer / primer layer / primed current collector produced by any of the methods described herein. Also provided herein is an electrochemical secondary cell comprising the electrode structure produced by any of the methods described herein. Also provided herein is an electrochemical energy storage device comprising such electrochemical secondary cell. In some embodiments, the electrochemical energy storage device may be a battery. In some embodiments, the electrochemical energy storage device may be a lithium-ion battery. In some embodiments, the electrochemical secondary cell and / or electrochemical energy storage device may comprise two such interlayers / primer layers / primed current collectors / electrode structures. For example, each of the anode and cathode may comprise an interlayer / primer layer / primed current collector / electrode structure produced by any of the methods of the present invention. These may have an electrolyte arranged between them. In some embodiments, the electrochemical secondary cell and / or electrochemical energy storage device may comprise one interlayer / primer layer / primed current collector / electrode structure produced by any of the methods of the present invention, and a second electrode, or interlayer / primer layer / primed current collector / electrode structure not according to the present invention. TTQFQ Also provided herein is a use of the printing apparatus as defined herein in a method for producing an electrode structure as described herein. Also provided herein is a use of a gravure printing process in a method for producing an electrode structure as described herein. The methods suitable for these uses are as described elsewhere herein, and may include, for example, adding a primer dispersion 7 comprising an electrically conductive additive and a binder to a first bath 1, arranging a first doctor blade 5 at a desired distance from the surface of a first engraved roller 3; applying a current collector 9 to the surface of a first transfer roller 11; and rotating the first roller set so that the primer dispersion 7 is taken up on a surface of the first engraved roller 3, excess primer dispersion, where present, is removed from the surface by the first doctor blade 5, and the primer dispersion 7 is transferred onto a current collector 9 surface by the first transfer roller 11. EXAMPLES Samples of primed current collector were prepared according to the claimed methods. The coating process involved pre-treatment of the current collector to increase the surface wettability and to remove surface oil (paraffinic or mixed oil may be used during the manufacturing process of rolling of the Al sheet) to enable good wetting of the solution and 5 achieve adequate levels of adhesion. The bare Al foil was unwound from the reel, subjected to pre-treatment, followed by coating and then drying. Pre-treatment involved vacuum plasma pre-treatment followed by CORONA treatment prior to gravure coating. The process parameter details are set out in the table below: Parameter Description / value Rewind Tension 40N Unwind Tension 10-50N Anilox(gravure) - cell volume 50 cc / m2 Dryer zone temperature 85°C Line speed 1-5 m / min Web guider Active Oxygen plasma treatment condition in vacuum 3 Gas flow - 250 O2 cm / min -3 Pressure-6.44x10 mbar CORONA treatment power - Pretreatment 2kW After gravure coating, the primer is dried in hot air oven at 85°C with the air velocity in the range of 9 to 10 m / sec (to distribute the hot air). All these processes are carried out in a roll-to-roll process. After drying, the primed foil was rewound into reel. The adhesion of primed 15 foil was carried out after drying using 3M tape manually and checked. The test method for surface wettability (Dyne test procedure) and adhesion test is as follows: Dyne test procedure: • Take a test marker with different Dyne level mentioned in pen • Hold the markers tip firmly against the corner of the subject material until the tip is saturated with test fluid. • Draw the marker across the Al foil sample in three passes and start the stopwatch. • Make sure the last ink remains wetted out on the Al foil surface and check the time period (in seconds) for which the ink does not shrink or tear apart. Adhesion test ■ Take the primed foil from reel with the size of A4 sheet ■ A strip of 3M tape about 5 cm long is adhered to the sample and the tape is rolled back and forth on the tape with a 10kg roller three times ■ The tape is tom off ■ If the primer is not peeled off with the tape, it is considered acceptable Viscosity measurements were made using HR30 Hybrid Rheometer from TA Instruments according to the following protocol and the results shown in Table 1: • Mount the disposable 25mm parallel plates onto the rheometer and perform calibrations. • Apply solution sample onto the lower plate with spatula (the sample needs to cover the entire plate surface). • Lower the upper plate to trim gap of 0.05 mm and remove extra sample from the rim of plates. • Lower the upper plate further to geometry gap of 1mm, close the environmental chamber. • Perform the rheological measurements and record the data, which can be plotted as viscosity against shear rate (both in log scale). • Test parameter: environmental chamber temperature is set to 25°C, the test is performed at shear rate range of 1 - 100 s’1 Table 1: Viscosity (cP) Shear rate (s4) 1 wt% solid (0.4 wt% carbon and 0.6 wt% binder) 1.4 wt% solid (0.8 wt% carbon and 0.6 wt% binder) 1 5958 65000 10 742 7421 15 475 4653 100 93 800 Figs. 2a-2d show photographs of three reels of primed current collectors in line with the present invention. Figs. 2a to 2c are from the same reel and show a blue foil. Fig. 3d is from a different reel and shows a yellow foil. The primer layer is applied to both sides of the current collector (Al foil). The reels of Figs. 2a-2c are shown in a rolled configuration. The outer portion of the sample is at the top of each of the figures. Samples for analysis were taken from the inner core, i.e. the inside of the roll. The reels of Figs. 2a-2c are each predominantly blue in colour. Some yellow / brown colour can be observed at the edges. The maximum loading was 0.45 GSM, with a minimum loading of 0.16 GSM and an average loading of 0.317 GSM with a range of 0.29 GSM. The loading was measured by cutting a sample of known size, and subtracting the theoretical weight of the bare Al foil. The result provides the coated mass and tolerance range. From each reel, 5 samples were cut suing a sample cutter for punching the samples. The sample cutter was rectangular, with a width of 6.9 cm and a length of 8.45 cm. The mass 46 deviation from the Al foil is higher than the coated weight. This increase is attributed to the primer layer thickness being much lower than the tolerance of the Al foil. The samples were collected in the machine direction (MD) taken from different sheets of the same reel. The reel of Fig. 2d is shown in unrolled condition. These had a yellow colour with little colour variation across the width. The notations A and B refer to different sides of the reel. No difference in performance based on the side of the reel has been noted. SEM imaging and analysis herein was performed using a JEOL(RTM) JSM-IT800 equipped with SEM center software and having integrated EDX capabilities. Fig. 3 shows results of surface elemental mapping of a primed current collector in accordance with the present invention, shown in Fig. 2b. This shows surface carbon agglomerations, attributed to sample preparation rather than the distribution of carbon in the primer layer. The strength of the EDX beam causes penetration further into the sample than the thickness of the primer layer and thus shows bulk Al from the current collector foil and not the primer layer. A sample of Figs. 2a-2c was taken, and a cross-section prepared and observed. The Al foil thickness 101 was 10 ± 0.5 pm according to the supplier specification. SEM measurements showed the Al foil thickness 101 was 10.24 pm, which matches the supplier specification. The primer layer was observable in the cross-sectional SEM images of Fig. 4a. Here, the primer layer is observable as a grey intermediate that of the bulk Al (lightest grey) and the background (black). For this sample, 5 measurements were made on Fig. 4a (i), 9 measurements were made on Fig. 4a (ii), 8 measurements were made on Fig. 4a (iii), and 8 measurements were made on Fig. 4a (iv) as indicated by the arrows. The average thickness at these four sites was 81.41nm ± 9.27 That these observations correspond to the layers indicated is supported by Fig. 4b. Here, EDX at high resolution of an SEM image (ii) shows clearly (i) a dense carbon layer (iii) above an Al layer (iv). The other carbon contribution is attributed to sample preparation / background. A sample was taken from the primed current collector of Fig. 2d (i). A cross-section was prepared and observed. The Al foil thickness tolerance is higher than the primer layer thickness. The primer layer was observed as an area of intermediate grey beside the dark Al foil layer, although in this sample it was much thinner than in the samples of Fig. 4. The primer layer was observable in the cross-sectional SEM images of Fig. 5a. Here, the primer layer is observable as a grey intermediate that of the bulk Al (lightest grey) and the background (black). The whitish spot observable in Fig. 5a (iv) is attributed to sputtering of the shield during the ion mill process before imaging. For this sample, 5 measurements were made on each of Figs. 5a(i)-(iv), as indicated by the arrows. The average thickness at these four sites was 53.52 nm ±8.25. That these observations correspond to the primer layers indicated is supported by Fig. 5b. Here, EDX at high resolution shows clearly (i) a dense carbon layer (iii) above an Al layer (iv). The other carbon contribution is attributed to sample preparation / background. Electrochemical analysis of an electrode structure formed from the primed current collector of Fig. 2d shows that the interlayer exhibited the same stability at temperatures between 30°C and 60°C, despite the extremely thin nature of the interlayer. See Figs. 6a-c. Accordingly, this demonstrates that interlayers roughly 5x thinner than those currently produced can be made according to the present invention, reducing overall materials usage and cost without sacrificing stability.

Claims

1. An electrode structure for use in an electrochemical cell, the electrode structure comprising:a current collector layer having a current collector surface;an electrode layer having an electrode surface that faces the current collector surface; andan interlayer located on the current collector surface and arranged between the current collector surface and the electrode surface;wherein the interlayer comprises:an electrically conductive additive comprising a tubular carbon material as a majority component by weight; anda binder comprising carboxymethyl cellulose; andwherein the interlayer has a maximum thickness of 150 nm or less.

2. The electrode structure according to claim 1, wherein the interlayer has an average thickness of less than 100 nm.

3. The electrode structure according to any one of the preceding claims, wherein the interlayer has a maximum thickness of 140 nm or less and / or a minimum thickness of 15 nm.

4. The electrode structure according to any one of the preceding claims, wherein the electrically conductive additive comprises 90 wt% or more of tubular carbon material, optionally wherein the electrically conductive additive consists essentially of the tubular carbon material.

5. The electrode structure according to any one of the preceding claims, wherein the tubular carbon material comprises or consists essentially of single-walled carbon nanotubes (SWCNTs) and / or multi-walled carbon nanotubes (MWCNTs).

6. The electrode structure according to claim 5, wherein the tubular carbon material consists essentially of single-walled carbon nanotubes (SWCNTs).

7. The electrode structure according to any one of the preceding claims, wherein the interlayer has a uniform thickness.

8. The electrode structure according to any one of the preceding claims, wherein the electrode layer is a polymer gel electrode layer.

9. The electrode structure according to any one of the preceding claims, wherein the current collector layer comprises or is aluminium foil.

10. A printing apparatus for producing a primer layer comprising an electrically conductive additive comprising a tubular carbon material as a majority component by weight, and a binder, wherein the primer layer has a maximum thickness of 150 nm or less; the printing apparatus comprising:a first bath for holding a primer dispersion comprising the tubular carbon material and the binder; and a first roller set comprising:a first engraved roller for taking up the primer dispersion from the bath onto a surface of the first engraved roller;a first doctor blade arranged laterally adjacent the first engraved roller for removal of excess primer dispersion from the surface of the first engraved roller to a desired thickness; anda first transfer roller arranged above the first engraved roller for applying the primer dispersion to a current collector surface of a current collector layer arranged on a surface of the first transfer roller;and wherein the first engraved roller and the first transfer roller are counter rotatable.

11. The printing apparatus according to claim 10, wherein:(i) the first engraved roller has a surface comprising a metal such as chrome, and / or(ii) the first transfer roller has a surface comprising an elastic material such as an elastomer, optionally wherein the elastic material is rubber.

12. The printing apparatus of any one of claims 10 to 11, wherein the first roller set further comprises a second roller arranged above the first transfer roller to allow a current collector layer to pass between the first transfer roller and the second roller, wherein the second roller has a surface comprising a non-elastic material such as a metal, optionally wherein the metal is chrome.

13. A method of producing an electrode structure according to any one of claims 1 to 9, the method comprising applying a primer layer corresponding to the interlayer, to the current collector surface using a gravure printing process.

14. The method according to claim 13, wherein the gravure printing process is an offset gravure printing process.

15. The method according to claim 14 or claim 15, wherein the gravure printing process uses the printing apparatus of any one of claims 10 to 12.

16. A method of applying a primer layer to a current collector, the primer layer comprising: an electrically conductive additive comprising a tubular carbon material as a majority component by weight, and a binder, and wherein the primer layer has a maximum thickness of 150 nm or less, the method comprising a gravure printing process.

17. The method according to claim 16, wherein the gravure printing process comprises the steps of:providing a gravure printing apparatus comprising a first bath and a first roller set, the first roller set comprising a first engraved roller, a first doctor blade arranged laterally adjacent the first engraved roller, and a first transfer roller arranged above the first engraved roller, wherein the first engraved roller and the first transfer roller are counter rotatable;adding a primer dispersion comprising the electrically conductive additive and the binder to the first bath, arranging the first doctor blade at a desired distance from the surface of the first engraved roller, and applying a current collector to a surface of the first transfer roller;taking up a portion of the primer dispersion from the bath onto a surface of the first engraved roller;removing any excess primer dispersion from the surface of the first engraved roller to a desired thickness using the first doctor blade;transferring primer dispersion from the surface of the first engraved roller to a surface of the first transfer roller; andapplying the primer dispersion from the first transfer roller to a current collector surface of a current collector layer.

18. The method according to claim 16 or claim 17, which uses the printing apparatus of any one of claims 10 to 12.

19. The method according to any one of claims 13 to 18, further comprising applying an electrode layer on the primer layer or interlayer.

20. An interlayer comprising an electrically conductive additive comprising a tubular carbon material as a majority component by weight, and a binder, and wherein the interlayer has a maximum thickness of 150 nm or less, wherein the interlayer is produced by a method comprising a gravure printing process.

21. An interlayer according to claim 20, wherein the method is a method according to any one of claims 13 to 19.

22. An interlayer according to claim 21, which is part of an electrode structure according to any one of claims 1 to 9.

23. An electrochemical secondary cell comprising the electrode structure of any one of claims 1 to 9, or the interlayer of any one of claims 20 to 22, or a primer layer applied to a current collector according to the method of any one of claims 16 to 19, or an electrode structure produced according to the method of any one of claims 13 to 15.

24. An electrochemical energy storage device comprising an electrochemical secondary cell according to claim 23.

25. Use of the printing apparatus according to any one of claims 10 to 12 in a method for 5 producing an electrode structure according to any one of claims 1 to 9, the method comprising:adding a primer dispersion comprising the electrically conductive additive and the binder to the first bath;arranging the first doctor blade at a desired distance from the surface of the first 10 engraved roller;applying a current collector to a surface of the first transfer roller; androtating the first roller set so that the primer dispersion is taken up on a surface of the first engraved roller, excess primer dispersion is removed from the surface by the first doctor blade, and the primer dispersion is transferred onto a current collector surface by the first 15 transfer roller.

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